Hard Reset HTC Desire HD

First method:

1. With the phone turned off, press and hold the Volume down button, and then briefly press the Power button.
2. Wait for the screen with the three Android images to appear, and then release the Volume down button.
3. Press Volume down to select Factory reset, and then press Power button. 
4. Press Volume up to start the factory reset.

Second method:

1. From the Home screen, press Menu and then tap Settings.
2. Scroll down the screen and then tap Privacy > Factory data reset.
3. On the Factory data reset screen, tap Reset phone, and then tap Erase everything.

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.